A root in The Etymologies of the 1930s glossed “round, about”, with various derivatives like N. o “about, concerning” and ᴹQ. osto/N. ost “city, town with wall round” (Ety/OS). Tolkien also considered variant forms ᴹ√OD and ᴹ√OTH, the latter glossed “fort” (EtyAC/OS). This root in The Etymologies is a later iteration of unglossed ᴱ√OSO [’OSO] from the Qenya Lexicon of the 1910s with derivatives like ᴱQ. ossa “wall and moat” and ᴱQ. ostar “township” (QL/71). It also had derivatives in the contemporaneous Gnomish Lexicon such as G. ost “enclosure, yard; town” and G. osta- “to surround with walls, build up; fortify, protect” (GL/63). This early root probably meant something like “✱enclosure”.
In the Quendi and Eldar essay from 1959-60, Q. osto and S. ost were instead derived from the root √SOT “shelter, protect, defend”, but this root has no other derivatives.
Neo-Eldarin: For purposes of Neo-Eldarin, ᴹ√OS “round, about” is too useful to discard, and I prefer it over the later root √SOT, though √(O)S-OT might be salvaged as an extension of √OS.
Tolkien gave this root in The Etymologies of the 1930s as ᴹ√TOR “brother” with derivatives like ᴹQ. toron and N. tôr of the same meaning (Ety/TOR). Hints of the roots continued use appear in the 1959 term Q. melotorni “love-brothers” for close male friends (NM/20). In notes from the late 1960s, Tolkien gave Q. háno and S. hanar as the words for “brother”, both from the root √KHAN. Nevertheless, I think it is worth retaining ᴹ√TOR to represent more abstract notions of “brotherhood” for the purposes of Neo-Eldarin, for “metaphorical” brothers as opposed to Q. háno/S. hanar for brothers by blood.